1. Field of the Invention
This invention relates to a device and a method for detecting smoke. It applies, in particular, to detecting a fire by means of the presence of fine particles or aerosols contained in the smoke, which makes it possible to reduce fire risks in premises where such devices are installed or where such methods are implemented.
2. Description of the Related Art
Two physical effects are mainly used to detect the presence of smoke, namely the diffusion of light by the smoke, dust or aerosols associated with it; and the change in the displacement speed of ions driven by an electric field as a result of this smoke, dust or aerosols.
In the first case, an optical smoke detector, as described for example in patent application FR 94 09473, comprises a chamber comprising apertures able to allow the smoke to enter into the chamber, a light source that can, when powered, emit a light beam into the chamber, a light receiver able to transform the reception of light into an electric reception signal, the interior of the chamber being weakly reflective in the spectral range of the light emitted by the source, the apertures being associated to chicanes able to prevent exterior light penetrating directly into the chamber and the source and the receiver being placed so as to able to prevent the light emitted by the source reaching the receiver directly.
Thus, in the absence of smoke containing particles large enough to diffuse the light emitted by the source, typically greater than 100 nm in size, the receiver receives practically no light and thus provides only a very weak electric reception signal, due solely to the light that penetrates from outside the chamber and to several possible reflections on the chamber's inner walls from the light emitted by the source.
In contrast, the presence of smoke or aerosols, associated with particles larger than 100 nm in the light beam, results in the appearance of diffused beams, some of which reach the receiver, possibly after being reflected on the chamber's inner walls, producing an electric reception signal.
Thus, this signal exceeding a predefined threshold allows an alarm to be triggered or sent.
Optical smoke detectors are especially sensitive to visible smoke, i.e. comprising large particles, but remain relatively insensitive to smoke comprised of fine particles, and thus almost invisible, which is emitted during the fire's first stages of development or in a hot fire.
In effect, when the smoke consists of fine particles that are too small or in too low a concentration, there is a very low level of light diffusion and the receiver receives an insufficient amount of light to allow the threshold for triggering or sending an alarm to be exceeded.
In the second case, an ionic smoke detector comprises a chamber in which two measurement electrodes are arranged between which qi charged ions are created or brought.
Applying a potential difference between these electrodes produces an electric field E which exerts a force F=qi×E on these ions; this produces a nominal electric current between the electrodes and in the external circuit that connects them, this electric current depending in particular on the amount of ions present in the chamber, the potential difference applied between the measurement electrodes, and the mobility of the ions.
Means of measuring this current are also provided, which provide a signal that can be used by processing means.
When particles associated with smoke enter the chamber, some of these particles become attached to the chamber's ions as a result of the electrostatic forces created by these ions; this reduces their mobility and has the effect of reducing the electric current.
If the voltage applied to the electrodes is low enough, typically between 5 and 30 volts, the nominal electric current is also low, typically between 10 μA and 100 μA, and the slowing down of the ions resulting from the presence of the particles is such as to reduce the amplitude of this current very substantially.
The processing means are arranged so as to allow an alarm to be triggered or sent when the current measured is below a predefined threshold.
Air ionization devices are more sensitive to combustion products, emitted during the initial development of fires or in hot fires; the size of these products can reach values of several tens of nm, or less, and thus allow alarms to be triggered earlier than optical devices, thus making it possible to limit the consequences of these fires.
Two approaches have been used to create ions in the measuring chamber: either by ionizing the air using a small radioactive source, as described, for example, in patent application FR 86 02567, or by creating an electric field stronger than the electric field for air breakdown, as described, for example, in patent U.S. Pat. No. 3,823,372.
The first approach is simple to implement and not very costly.
For example, a source of α particles comprised of Am-241 with activity between 0.1 and 1 microcurie is used; these particles can cross a distance of the order of centimeters in the air and thus ionize the volume passed through.
However, although this solution makes it possible to detect fires early and thus reduce their consequences, it is facing increasing challenges, either from users themselves, who are reluctant to increase radioactive sources in their premises, or from manufacturers' sales departments, who are confronted by negative reactions from their customers, or from regulations.
With regards to the second approach, various solutions have been proposed for ionizing the air. These approaches use the fact that, by applying a potential difference above a certain threshold Vs between two electrodes, one can initiate a process of electrical discharge and thus create ions.
The value Vs depends on several parameters, such as the nature of the gas between the electrodes, the pressure of the gas separating them, the distance between the electrodes and their shape, the presence of dust or humidity, etc.
In the air, this threshold is considered to be approximately 330 V for distances between electrodes of the order of micrometers, distances too small to be used directly in a smoke detector, which means that voltages of several kilovolts must be used to create this ionization.
However, values such as these cannot be used for polarizing the measurement electrodes since the high speed of the ions resulting from this would lead to a very high nominal electric current and these ions would cross the measurement chamber in a very short period of time.
As a result, the changes in this current because of the presence of particles associated with smoke would be so small that they would be difficult to detect.
To overcome this obstacle, various proposals have been made to increase the interaction time between the ions and the smoke particles.
A first approach has been to use a measurement chamber polarized by a weak voltage, into which ions produced in an ionization chamber polarized by a high voltage are transferred, by means of a weak current of air, and thus to have a low nominal current.
An example of such a solution is described in patent application FR 96 03296.
In a second approach, reflective elements have been introduced between the electrodes of an ionization chamber polarized by a high voltage, so as to increase the interaction time.
An example of such a solution is described in patent application U.S. Pat. No. 3,932,851.
These alternative solutions utilize high electric voltages which, however, result either in detectors that have relatively low detection sensitivities due to the very fact of using high voltages, reducing their advantages, or in devices that are mechanically complex, fragile and expensive.
In addition, the response of these detectors is also influenced by parameters such as variations in ambient gas pressure or in temperature, thus requiring compensation devices such as, for example, described in patent EP-236 223, to be used as well.
For these reasons there have been no major industrial developments of these alternative solutions.
In order to allow both small- and large-sized particles to be detected, and thus to cover the risks associated with the various phases of a fire, combining ionic smoke detection devices, well suited to detecting small particles, and optical smoke detection devices, well suited to detecting larger particles, inside the same detector, has been proposed, for example in patent U.S. Pat. No. 4,469,953.
In this case, so as to allow chambers to be constructed that are electromagnetically shielded and optically weakly reflective, and also not very expensive, it has been proposed, as described, for example, in patents U.S. Pat. Nos. 4,469,953 and 4,225,860, to produce the electrodes or shields using conductive plastics rather than using metal parts.
Clearly, the ionic portion of these detectors has the same drawbacks as detectors that operate solely in ionic mode, as described above.